1. Field of the Invention
The present invention relates to assemblies for initiating explosives by means of shock tube/detonator units, and to low-energy shock tubes and detonators adapted for use in such assemblies. The invention relates also to primer assemblies containing shock tube/detonator units for use in the non-electric initiation of cap-insensitive explosives, and more particularly for use in the delayed initiation of deck-loaded explosive charges by means of a single detonating cord downline.
2. Description of the Prior Art
A shock tube, or shock tubing, or a shock fuse, also known as fuses, impulse propagating tubing, signal transmission line, etc., is a low-energy shock tube which includes an elongated, hollow tube forming a gas channel, the inner surface of which tube is coated with a reactive substance, e.g., a thin layer of a detonating or deflagrating explosive composition. When the fuse is initiated at one end, a low-energy gaseous pressure, percussion or shock wave, pulse or signal is created, and propagated within the gas channel from one end of the tube to the other to actuate a shock tube detonator attached to the remote end of the tube. These definitions of the shock tube and the energy generated in the gas channel within the shock tube walls are used in different publications, but they all basically mean the same thing. As described in U.S. Pat. No. 3,590,739, the amount of reactive material on the inner surface of the fuse is so small that the tube is not perforated. Initiating of the shock tube is accomplished by such devices as a detonator, a delay detonator as described in U.S. Pat. No. 3,987,732, a percussion device as described in U.S. Pat. No. 5,327,835, an electric spark and detonating cords.
The need to reinforce the wall of the shock tube for one advantage or another has been recognized. Laminated, or multi-ply shock tubes are described, for example, in U.S. Pat. Nos. 4,328,753, 4,493,261, and 4,607,573. In these examples, each tube layer is made of a different plastic material to confer a required property, such as an outer layer made of a material having mechanical strength, and an inner layer of a material, such as Surlyn.RTM., a polyethylene ionomer, to which the powdered reactive material, applied to the inner surface of the tubular composite, is adherent. Attempts to use a low-energy detonating cord (LEDC) in the middle and lower loading range to initiate shock tubes of any construction of wall thickness of about 1 millimeter and thicker were unsuccessful.
Shock tube/detonator units, wherein one end of the tube fits into a shock tube type detonator (see, for example, U.S. Pat. No. 3,817,181) are well-known in the blasting art, e.g., for use in initiating primers, as shown in U.S. Pat. No. 4,527,482. The latter patent makes reference to the prior art wherein such units, known commercially as Nonel Primadet.RTM., a non-electric shock tube with a detonator, and HD Nonel Primadet.RTM., a heavy-duty-type shock tube, where one end of the fuse or shock tube of these units, e.g., a 30-inch length of shock tubing, is crimped to a delay blasting cap (i.e., detonator) are used. The external end of the shock tube is initiated by, e.g., HD Primaline.RTM., a heavy duty 1.6 grams per meter (7.5 grains per foot) detonating cord manufactured by the Ensign-Bickford Co. or by other higher-energy detonating cord. It is known to those skilled in the art that this cord will initiate the shock tube, especially when knotted in a double-wrapped square knot where its output is at least doubled at the knot. The same patent proceeds to teach how an external adapter to a primer wherein the adapter has different tunnels, is used to accommodate shock tubes/detonator units with different detonating cords. It is said that the smallest cord of 1.5 to 1.6 grams per meter (7 to 7.5 grains per foot) is to be confined with the shock tube in one tunnel, and higher-energy detonating cords of at least 18 grains per foot (3.8 grams per meter) in other tunnels for initiating shock tube/detonator units in the deck loading technique of blasting wherein a primer is positioned in each of a number of separated decks of cap-insensitive explosive, and wherein the primers are connected by a single downline detonating cord threaded through the adapter which is attached externally to each primer. The detonator of the shock tube is seated in an axial cavity in the primer, and the shock tube itself, called a pigtail, is inserted into one of the external adapter tunnels, depending on the amount of energy transmitted by the detonation from the detonating downline via the adapter, to initiate the shock tube. Sometimes a piece of another detonating cord, used to boost the detonation of the detonating cord, is inserted with the shock tube in the same tunnel of the shock tube to assure initiation. The purpose of this external adapter in U.S. Pat. No. 4,527,482 is that the recommended detonating cords, even the smallest mentioned, would either initiate the primer itself, or destroy it, or initiate the delay detonator instantly if inserted in the primer axial internal cord tunnel, thereby circumventing the required delay timing of the decks in the hole.
There are other patents addressing the same issue of placing the detonating cord outside the primer in a protective casing, tubes, or carriers and providing means of coupling the shock tube of the shock tube/detonator unit to the detonating cord. U.S. Pat. No. 4,133,247 is of particular interest since it states the problem of using detonating cords inside the primers. It uses 2.5 to 12.8 grams per meter (12 to 60 grains per foot) for reliable initiation of the shock tube when placed in the same external tunnel of a protective carrier for the primer.
The primer assembly described in U.S. Pat. No. 4,718,345 allows a detonating cord to be threaded through an axial cord tunnel of the primer by selecting a detonating cord in the middle to the low range of LEDC for a downline. Because of the low explosive loading of the preferred LEDC, i.e., about 0.5 gram per meter (2.3 grains per foot) of cord length, the mild initiation impulse from the output of the downline is amplified and relayed to the delay detonator seated in a cavity in the primer via an explosive coupler which is in initiating proximity to a percussion-sensitive element in the detonator, without adversely affecting the primer, or the detonator or the explosives in the bore hole. However, neither U.S. Pat. No. 4,718,345, nor any other patent or publication teaches how to use low-energy detonating cords of less than 1.5 grams per meter (7 grains per foot) to initiate shock tube systems without the added expense of a coupler, or a detonator.